a via
/ VEE-uh /
A via is a tiny plated hole that lets a trace jump from one copper layer of the board to another — an electrical staircase between floors. On a two-layer board, when a trace running on the top must continue on the bottom (to dodge another trace, say), it ends at a via: a small drilled hole whose inner wall is plated with copper, so current flows down through the barrel from the top trace to the bottom trace. Without vias, every connection would have to live on a single layer, and any real board would be impossible to route.
The commonest type is a through via: drilled straight through the entire board and copper-plated, connecting top, bottom, and any inner layers it passes. On multilayer boards two clever variants exist. A blind via starts on an outer layer and stops at an inner layer (it does not go all the way through, so it is blind from the far side). A buried via connects only inner layers and never reaches either surface, completely hidden inside the stack. These let dense boards pack connections without wasting outer-layer space, but they cost more to manufacture.
Why it matters: vias make multilayer routing possible, but each one has a price. A via has real resistance and inductance, so a fast or high-current signal does not love passing through one — for heavy current you use several vias in parallel, and for fast edges you keep via count low. Vias also eat board space and can punch holes in your ground plane if scattered carelessly. A special trick, the thermal via, is a cluster of vias under a hot chip used to conduct heat down to a copper plane. Used well, vias are essential; sprinkled thoughtlessly, they quietly degrade a board.
A trace runs along the top layer, hits a via, dives through the board, and continues on the bottom layer past an obstruction. Under a hot voltage-regulator pad, a 3-by-3 grid of small vias carries heat down to the ground plane to spread it out.
An electrical staircase between layers — handy, but each one adds resistance and inductance.
Blind and buried vias sound elegant but multiply fabrication cost and lead time. For most boards, plain through vias are cheaper and perfectly fine; reach for the fancy kinds only when a dense, fine-pitch design genuinely forces your hand.